Radiotherapeutic apparatus generally comprises a radiation source which produces a beam of therapeutic radiation, i.e. radiation at a suitably high energy level to cause damage to tissue through which it passes. The beam is then collimated and directed towards a patient. This collimation of the beam seeks to limit its lateral extent so that it selectively irradiates a tumour within the patient and thereby causes harm to the tumour.
Generally, the radiation source is supported on a gantry extending from a mount which is rotated during irradiation so as to direct the beam towards the patient from a variety of directions. This means that the portion of healthy tissue through which the beam passes in order to irradiate the tumour varies with time, and the total radiation dose delivered to any particular volume of healthy tissue is thereby minimised. As the radiation source rotates around the patient, the collimation of the beam may be changed, for example to reflect the changing projected shape of the tumour (in a conformal arc-type therapy), or to lay down differing dose distributions (in intensity modulated radiotherapy applications). In the latter example, the dose rate or intensity of the beam may be adjusted as the treatment progresses in order to create a three-dimensional prescription that is individual to the patient concerned. Thus, as the treatment progresses, the gantry angle, the collimation, and the beam intensity may all be varying dynamically. These must obviously be monitored for error.
Where an error beyond an acceptable threshold is detected, the radiation source is deactivated in order to prevent harm being caused to the patient. A certain threshold of error must be permitted, simply because there will inevitably be some degree of lag, inertia and other measurement error in the system, and hence a zero threshold would have the potential to stop all treatment or to make treatment so slow that it becomes inefficient.